Spark plug

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PROBLEM TO BE SOLVED : To provide a spark plug in which voltage resistance characteristics and mechanical strength are improved by reducing a drop rate relative to a normal temperature, even at a high temperature. SOLUTION : A spark plug 1 comprises an insulator 3 which contains an Al component, an Si component, a periodic table group 2 component based on the IUPAC 1990 recommendation and a rare earth component in a content ratio satisfying conditions (1) and (2) and is made of an alumina group sintered body that contains alumina particles and a glass phase including a crystal phase satisfying a condition (3). The condition (1) is 0.25≤content ratio (mass %) converting the Si component into oxide≤1.25, the condition (2) is 1.35≤[content ratio converting the Si component into oxide/(content ratio converting the group 2 component into oxide/content ratio (mass %) converting the rare earth component into oxide)]≤7.5, and the condition (3) is that at least one of an aluminate crystal phase and a silicate crystal phase of which the particle size is ranged from 20 to 350 nm is contained. COPYRIGHT : (C)2013, JPO&INPIT